System for utilizing impact induced transitions in a microwave resonant medium



Dec. 20, 1960 L. E. NORTON 2,965,795

SYSTEM FOR UTILIZING IMPACI` INDUCED TRANsI-IIoNs IN A MICROWAVEREsoNANI MEDIUM Filed Aug. 1, 1955 5, /f/l.. M Il1 a... m /a H Illu d A1, w .i mz .w w+ I4/ll.| n@ u@ J in 4. M 52wk 7m Mw INVENTOR. LDWELI. E.Num DN BY g Z0 QZ Unite States Patent SYSTEM FOR UTILlZlNG IlVilACTINDUCED TRANSITINS IN A MICROWAVE RES()- NANT MEDEUM Lowell E. Norton,Princeton, NJ., assigner to Radio Corporation of America, a corporationof Delaware Filed Aug. 1, 1955, Ser. No. 525,442

31 Claims. (Cl. S15-5.35)

This application is a continuaton-in-part of my copending applicationSerial No. 497,762, filed March 29, 1955, now abandoned.

This invention relates generally to systems employing microwave resonantmedia and particularly to improved methods of and means for increasingthe intensity of a spectral line exhibited by a microwave resonantmedium by an impact phenomenon which produces a preferred nuclearorientation of particles in certain energy levels` According to theinvention the particles having the preferred orientation may be used inmicrowave spectroscopy and frequency stabilization systems, or formicrowave energy generation or amplification.

Spectral lines in the microwave region of the radiofrequency spectrumare relatively weak in intensity primarily because of the smallpopulation differences of atoms or molecules in the various quantumenergy levels of interest. At 25,000 megacycles per second, for example,where ammonia is the resonant medium employed,

the ratio of the two energy levels involved differs from unity by onlyabout 4 10*3. The ratio of the populations of the molecules in theseenergy levels in then [l-i-(4X10-3N. For transitions where the nuclearspin of Na23 is involved, a spectral line is exhibited at about 1,771megacycles per second, and the per unit population difference of theenergy levels concerned is approximately 2.8Xl-4. The correspondingratio of the populations of the sodium atoms is [1+(2.8 10*4)]. Forother resonant media and frequencies the per unit population diiferencesof energy levels of interest is even less desirable.

An object of the invention is to provide an improved method of and meansfor utilizing a microwave resonant medium.

Another object of the invention is to increase the intensity of aspectral line of a microwave resonant medium by a particle impactphenomenon.

Another object of the invention is to provide an improved method of andmeans for utilizing a microwave resonant medium for microwavespectroscopy and/or frequency stabilization.

Another object of the invention is to provide an improved method of andmeans for utilizing a microwave resonant medium for generatingelectrical energy.

A further object of the invention is to provide an irnproved method ofand means for utilizing a microwave resonant medium for amplifyingelectrical energy.

A further object of the invention is to improve the signal-to-noiseratio of systems in which microwave resonant media are employed.

A still further object of the inventionv is to produce a spectral lineof increased intensity and reduced bandwidth.

The foregoing objects and advantages are achieved in accordance with theinvention by enhancing the small population difference heretoforementioned which is due to the initial thermal equilibrium condition ofparticles of a microwave resonant medium. The small populationdifference is improved by selecting as preferred at least one allowedenergy and angular momentum state of the atom or molecule. Preferentialpopulation of the preferred quantum state or states results inintensification of the selected spectral line.

Briefly, the preferential energy state population referred to above isachieved by subjecting a microwave resonant medium such as a gas, avapor, or a solid to a particle beam which moves at a selected andcontrolled velocity. The beam particles collide with the atoms of theresonant medium and, with the beam velocity properly controlled, causecertain permitted energy level transitions to occur between magneticsubstates of at least a pair of quantum energy states. The magneticsubstates are made definite by applying a weak magnetic eld of definitedirection to the resonant medium. These impact induced transitions arefollowed by spontaneous drop-down transitions from magnetic substates ofthe higher energy state to magnetic substates of the lower energy state.The net result of the impact induced and drop-down transitions is thatthe populations of certain magnetic substates of the lower energyquantum state are enhanced at the expense of the populations of othermagnetic substates of the same lower energy level. The enhancing of thepopulation of one magnetic substate at the expense of another magneticsubstate to a large extent improves the initial thermal equilibriumcondition mentioned previously, and enables the microwave resonantmedium to be used more efficiently for microwave spectroscopy, frequencystabilization, or for the generation or amplification of microwaveenergy.

Since the intensity of the spectral line of interest is greatly enhancedby means of the impact phenomenon briefly described above, in someinstances it may be desirable to sacrifice a portion of the increasedline intensity for a reduction in the spectral line bandwidth.Therefore, in situations where the microwave resonant medium is a gas ora vapor, the Doppler-breadth of the spectral line may be reduced, inaccordance with a further feature of the invention, by mixing a buffergas with the microwave resonant gas or vapor. However, theDopplerreduced bandwidth line intensity still is considerably greaterthan would be the case if impact induced transitions were not caused tooccur.

The invention will be described in detail with reference to theaccompanying drawing in which:

Figure 1 is a schematic sectional diagram of apparatus for producingpreferential populations of certain quantum energy states, according tothe invention; and

Figure 2 is an energy level diagram of transitions which are permittedto occur between various quantum energy states in accordance with theinvention.

Structure Referring to Figure 1, a typical embodiment of the inventionincludes a non-magnetic envelope 11, for example, a glass envelope,containing therein a microwave resonant medium. The resonant medium,further by way of example, may comprise a gas such as ammonia, vaporssuch as sodium 23 or cesium 133 (uids), or materials such as CHI'CHI(methyl iodide) or I2 (iodine) which are solids at room temperature andpressure.

In the present example it is assumed that the microwave resonant mediumchosen is sodium (Na23) at a vapor pressure not greater than 102millimeters of mercury. Preferably the vapor pressure is between l04 and10-6 millimeters of mercury. The sodium vapor is provided by a reservoiror source 13 which is connected to the envelope 1l via a conduit 15. Aheater 17, supplied with current from a source (not shown), is providedfor controlling the temperature and thereby the pressure of the sodiumvapor in the reservoir or source 13.

The envelope 11 also contains means for generating a particle beamwherein the particles move in a predetermined direction at closelycontrolled velocities. In the embodiment illustrated a cathode 19 emitselectrons at thermal velocities. An apertured accelerating and beamforming electrode 21 is spaced from the cathode 19 and is maintained, bya battery 23 or other D.C. source, at a potential which is positive withrespect to the cathode potential, for example, +100 volts. A reasonablyintense electron beam is thereby produced. This electron beam then issubjected to a decelerating field and enters an electron permeablecavity resonator Z5. The dccelerating field is produced by means of abattery 27 or equivalent source connected between the acceleratingelectrode 21 and the resonator 25. The decelerating field in the presentexample may be 97.9 volts. A relatively intense |-2.l volt electron beamthus is produced and caused to enter the resonator 2S.

In the event that it is necessary to focus the electron beamconventional electrostatic or magnetic focusing structure, per se wellknown, may be employed.

The electron permeable resonator 25 may be a wire mesh cage formed froma non-magnetic material such as copper or aluminum. The resonatorpreferably is cylindrical in shape and preferably is operated in the TEMmode at a frequency which is determined by the difference in energylevels of two selected magnetic nuclear substates. The only requirementimposed on the cage mesh size is that the mesh must be transparent toimpacting electrons yet be opaque to electromagnetic fields set uptherein. For Na23 the frequency to which the resonator 25 may be tunedis 1771 megacycles per second, whereas for Cs133 the operating frequencymay be 9192.6 megacycles per second. Input and output couplings to theresonator 25 are afforded by coupling loops 29 and 31, respectively.

A weak D.C. magnetic field, approximately 0.1 gauss, for example, isimpressed on the vapor by means of current through a coil 33 locatedoutside the envelope. The magnetic lines of force H extend in thedirection of motion of the electron beam.

Theory of operation The operation of the structure described in theforegoing paragraphs is believed to be as follows. The sodium vaporatoms initially are in a 381/2 ground state. Considering that thenuclear spin I is 3/5, the permitted magnetic substates MF of the F=lenergy level of the ground state and the F :2 level of the 3P1/11excited state are as shown in Figure 2. For the case where eV, theimpacting energy of the electron beam, is just equal to the energydifference between the 3P and 3S states, the electrons, after collisionswith sodium vapor atoms, have zero velocity and energy. The angularmomentum transferred from the impacting electrons to the sodium atoms isat right angles to the motion of the electron stream, and the Weakmagnetic field H produced by the coil 33 resolves the degeneracy of themagnetic substates. Thus, the transferred angular momentum also isnormal to the magnetic field H. The important conclusion which followsis that only transitions which follow the interval rule can be inducedby the impact of electrons moving in the direction of the magnetic fieldH.

With the electron energy eV=2.l volts, the electrons entering theresonator 25 cause impact induced transitions to occur between the F=llevel of the 381/2 ground state and the F=2 level of the 3P1/2 state.The SP1/2 yand SP3/1 states are the excited states which give rise tothe well known sodium D lines.

Using the interval rule Am=0, only the transitions shown in Figure 2 arepermitted. Electron impact induced transitions from the magneticsubstate M F=-l of the F=l level of the 381/2 ground state to themagnetic substate MF=1 of the F=2 level of the SP1/2 excited state arefollowed by drop-down" transitions to the MF=l or MF=0 substates of theF=l level of the 381/2 state. Similarly, impact induced transitions fromthe MF=11 substate of the F=l level of the 381/2 ground state to theMF=+1 substate of the F=2 level of the SP1/2 excited state are followedby drop-down transitions to the MF=+1 and MF=0 substates of the F =llevel of the 381 /2 state. Also, impact induced transitions from theMF=0 magnetic substate of the F=l level of the 381/2 ground state to theMF=O substate of the F=2 level of the SP1/2 excited states are followedby drop-down transitions to any of the MF=1, MF=0, MF=+1 substates ofthe F =1 level of the 381/2 ground state.

As a result of the above-described impact induced transitions and theensuing drop-down transitions, the number of sodium atoms in the MF=0magnetic substate of the F=l level of the 381/2 state is enhanced at theexpense of the MF=1 and MF=+1 magnetic substates of the F =l level ofthe 381 /2 state. The new population distribution comprises a preferrednuclear orientation in which the original population difference of theMF=0 substates of the F=l, F=2 levels of the 381/2 state is enhanced byseveral orders of magnitude. The MF==0 substate of the F=2 level of the381 /2 ground state is less densely populated than the MF=0 substate ofthe F=l level of the ground state both before and after the impact orcollision process is caused to occur. However, as a result of the impactphenomena the population difference between these substates is enhancedby a factor of about 103 which results in a spectral line of greatlyincreased intensity.

Since the intensity of the spectral line is increased in accordance withthe method described above, the signalto-noise ratio of the system inwhich the apparatus of Figure l is employed is considerably improved. Insystems for providing microwave spectroscopy or frequency stabilizationthe apparatus may be connected into such systems by means of theresonator input and output coupling loops 29 and 31, respectively. Thefrequency of the monochromatic excitation applied to the resonator 25via the input coupling loop 29 is determined by the e11- ergy levels ofthe M1=-=0 substates of the F=l and F=2 levels of the 381/2 state. Asindicated previously, for Na23 this frequency is 1,771 megacycles persecond. For other spectral lines or resonant media the resonator tuningand the frequency of excitation energy are different.

In situations where microwave amplification or oscillation generationare desired, rather than microwave spectroscopy or frequencystabilization, it is necessary only to select other magnetic substatesfor utilization. In both cases, i.e., amplification and oscillationgeneration, it is necessary that the selected magnetic substate of theupper energy level of the ground state be more densely populated thanthe selected magnetic substate of the lower energy level of the groundstate. Preferably, the population inequality of these two substatesshould be as great as possible.

It is important to note at this point that the population condition foramplification or oscillation generation is the reverse of the populationcondition for microwave spectroscopy or for ordinary absorption spectralline frequency stabilization. In those cases i.e., spectroscopy orfrequency stabilization, the upper energy level of the ground state isless densely populated than the lower energy level of the ground state.

In general the greatest population increase in the upper energy level ofthe ground state occurs for (or near) the magnetic substate where I isthe angular momentum of the particle (a sodium atom in the presentexample) and I is the nuclear spin of the atom.

The greatest population decrease in the lower energy level of the groundstate occurs, for (or near). the mag,- netic substates Therefore, byselecting the magnetic substates so that the population of the upperenergy level of the ground state is greater than the population of thelower energy level of the ground state, the resonant medium coherentlyradiates or emits rather than absorbs energy at a frequency at which themedium is resonant. When the medium is in this radiative or emissivecondition it may be said to have negative attenuation.

With the intensity of the coherent radiation sufficient to exceed theusual cavity resonator losses, the overall attenuation of the structureis negative and the structure ampliiies microwave energy applied to themedium at a frequency at which the medium is resonant. Amplified outputenergy is. derived from the resonant medium at a rate equal to the sumof (l) the rate at which energy is applied to the resonator and (2) therate at which electromagnetic energy is generated within the resonatorby the quantum transition process. Inasmuch as the magnetic substatesselected for amplification purposes are different from the magneticsubstates selected for spectroscopy or frequency stabilization, thefrequency of the input energy applied to the medium (the sodium) vapor)for amplification is close to, but not the same as, the 1,771 megacyclesper second frequency referred to previously. This is because theexcitation frequency is determined by the energy levels of the selectedmagnetic substates. The frequency of the input energy applied to thevapor via the coupling loop 29 may differ from the 1,771 megacycles persecond frequency by one to several megacycles, depending on theparticular substates chosen.

In the event that oscillation generation rather than amplification isdesired, the input coupling loop 29 may be omitted and the cavityresonator 25 strongly decoupled with respect to its association loadcircuit (not shown). The magnetic substates selected for oscillationgeneration may be the same as those selected for amplification purposes.In such case a spontaneous build-up of electromagnetic oscillationsoccurs within the resonator 2S at a frequency at which the sodium atomsare resonant. With the electromagnetic energy withdrawn from theresonator at a rate which is slower than the rate of build-up of theoscillations, the device generates microwave energy which may be coupledtherefrom by the output coupling loop 3l. In some instances it may bedesirable to improve the oscillation generation capabilities of thedevice by operating the device at a reduced temperatuer to increase itsQ.

While the embodiment of the invention described above teaches the use ofelectrons as the impacting particles, it is emphasized that otherparticles also may be employed, for example, ions, a molecular beam, orneutrons.

Doppler breadth reduction In view of the fact that the impact phenomenonaffords a spectral line of greatly increased intensity, in someinstances it may be desirable to trade some of the increased lineintensity for a line of effectively narrower bandwidth. In theseinstances, and in accordance with a further feature of the invention, asmall quantity of buffer gas, for example, a noble gas such as helium orargon, is introduced into the envelope 1l and mixed with the resonantgas or vapor. The partial pressure of tne resonant medium preferably isless than 10c-2 millimeters of mercury, as described previously, whereasthe partial pressure of the noble gas preferably is several orders ofmagnitude greater. The particles of the noble gas effectively provide along diffusion time for the particles or molecules of the microwaveresonant medium before they strike the resonator walls. This is becausethe resonant particles or molecules collide with the noble gasparticlesbefore they strike the resonator walls. Internas of thewavelength associated with magnetic dipole transitions of the particlesor molecules of the resonant medium the mean free path is small.

In considering the overall effect of inter-particle collisions it isnecessary to average overall particles in all velocity classes. Ingeneral the resonant atoms or molecules of all classes contribute tospectral line components at discrete frequencies. However, the resonantatoms or molecules of all classes also contribute to one common spectralline component at the non-Doppler shifted frequency. The result is aspectral line of essentially normal Doppler breadth with a sharp, narrowbandwidth line superimposed thereon which is free from Dopplerbroadening.

What is claimed is:

l. Microwave apparatus comprising, a microwave resonant medium capableof exhibiting discrete energy level transitions between quantum energyrotational states with resultant signal translation at at least oneresonant transitional frequency distinctively characteristie of saidmedium, means for producing a beam of particles having energiessufficient to induce said rotational transitions but insufficient toionize said medium, means for applying a magnetic field to saidmicrowave resonant medium to resolve the degeneracy of said rotationalstates, means for causing said beam particles to collide with saidmicrowave resonant medium to induce said transitions with said resultanttranslation, and means for providing electrical coupling to said signaltranslation from said microwave resonant medium.

2. Microwave apparatus comprising, a microwave resonant medium capableof exhibiting discrete energy level transitions between quantum energyrotational states with resultant signal translation at at least oneresonant transitional frequency distinctively characteristic of saidmedium, means for producing a beam of particles having energiessufficient to induce said rotational transitions but insufficient toionize said medium, means for applying a magnetic field to saidmicrowave resonant medium with the magnetic lines of said field parallelto the path of said beam to resolve the degeneracy of said rotationalstates, means for causing said beam particles to move at predetermineduniform velocities whereby said uniform velocity particles collide withsaid microwave resonant medium to induce said transitions with saidresultant translation, and means for applying microwave input energy toand coupling microwave output energy from said medium at a frequency forwhich said medium is resonant to utilize said translation.

3. Microwave apparatus for producing an intensified spectral linecomprising, a microwave resonant medium capable of exhibiting energylevel transitions between quantum energy rotational states withresultant spectral line intensification at at least one resonanttransitional frequency distinctively characteristic of said medium,means for producing a beam of particles having energies suflicient toinduce said rotational transitions but insumcient to ionize said medium,means for applying a magnetic field to said microwave resonant mediumwith the magnetic lines of said field parallel to the path of said beamto resolve the degeneracy of said rotational states, means for causingsaid beam particles to move at predetermined uniform velocities wherebysaid uniform velocity particles collide with said microwave resonantmedium to induce transitions with said resultant line intensification,means for applying microwave input energy to said medium at a frequencyfor which said medium is resonant, and means responsive to saidintensitied spectral line for deriving from said medium output microwaveenergy at said resonant frequency.

4. Microwave apparatus comprising, a microwave resonant medium, a noblegas mixed with said microwave resonant medium, means for producing abeam of par' ticles, means for causing said beam particles to collidewith said microwave resonant medium, means for applying a magnetic eldto said microwave resonant medium with the magnetic lines of said fieldparallel to the path of said beam, means for applying microwave inputenergy to said medium at a frequency for which said medium iS resonant,and means for deriving from said medium output microwave energy at saidresonant frequency.

5. Microwave apparatus comprising, a microwave resonant medium capableof exhibiting discrete energy level transition between quantum energyrotational states with resultant signal translation at at least oneresonant transitional frequency distinctively characteristic of saidmedium, means for producing particles having energies sufficient toinduce said rotational transitions but insufcient to ionize said medium,means for applying a magnetic field to said microwave resonant mediumwith the magnetic lines of said field parallel to the path of said beamto resolve the degeneracy of said rotational states, means forsuccessively accelerating and decelerating said particles to form anintense low velocity particle beam whereby said beam particles collidewith said microwave resonant medium to induce said transitions with saidresultant translation, means for applying microwave input energy to saidmedium at a frequency for which said medium is resonant, and meansresponsive to said translation for deriving from said medium outputmicrowave energy at said resonant frequency.

6. Microwave apparatus comprising, a microwave resonant gas capable ofexhibiting energy level transitions between quantum energy rotationalstates with resultant spectral line intensification at at least oneresonant transitional frequency distinctively characteristic of saidgas, means for producing a beam of particles having energies sufficientto induce said rotational transitions but insufflcient to ionize saidmedium, means for applying a magnetic field to said microwave resonantgas with the magnetic lines of said field parallel to the path of saidbeam to resolve the degeneracy of said rotational states, means forcausing said beam particles to move at predetermined uniform velocitieswhereby said uniform velocity particles collide with said microwaveresonant gas to induce transitions with said resultant lineintensifcation, means for applying microwave input energy to said gas ata frequency for which said gas is resonant, and means responsive to saidintensified spectral line for deriving from said gas output microwaveenergy at said resonant frequency.

7. Microwave apparatus comprising, a microwave resonant vapor capable ofexhibiting energy level transitions between quantum energy rotationalstates with resultant spectral line intensification at at least oneresonant transitional frequency distinctively characteristic of saidvapor, means for producing a beam of particles having energiessufficient to induce said rotational transitions but insufiicient toionize said medium, means for applying a magnetic field to saidmicrowave resonant vapor with the magnetic lines of said field parallelto the path of said beam to resolve the degeneracy of said rotationalstates, means for causing said beam particles to move at predetermineduniform velocities whereby said uniform velocity particles collide withsaid microwave resonant vapor to induce transitions with said resultantline intensification, means for applying microwave input energy to saidvapor at a frequency for which said vapor 1s resonant, and meansresponsive to said intensified spectral line for deriving from saidvapor output microwave energy at said resonant frequency.

8. Apparatus as claimed in claim 7 wherein said vapor 1s sodium and saidparticles are electrons.

9. Microwave apparatus comprising a microwave resonant material capableof exhibiting energy level transitions between quantum energy rotationalstates with resultant spectral line intensification at at least oneresonant transitional frequency distinctively characteristic of saidmaterial which is solid at room temperature and pressure, means forproducing a beam of particles having energies sufficient to induce saidrotational transitions but insufficient to ionize said medium, means forapplying a magnetic field to said microwave resonant material with themagnetic lines of said field parallel to the path of said beam toresolve the degeneracy of said rotational states, means for causing saidbeam particles to move at predetermined uniform velocities whereby saiduniform velocity particles collide with said microwave resonant materialto induce said transitions with said resultant translation, means forapplying microwave input energy to said material at a frequency forwhich said material is resonant, and means responsive to saidtranslation for deriving from said material output microwave energy atsaid resonant frequency.

l0. Apparatus according to claim 2 wherein said medium has a pressurenot greater than 10-2 millimeters of mercury.

1l. Apparatus as claimed in claim 12 wherein said particles areelectrons.

12. Microwave apparatus comprising, an envelope containing a microwaveresonant medium capable of exhibiting discrete energy level transitionsbetween quantum energy rotational states with resultant signaltranslation at at least one resonant transitional frequencydistinctively characteristic of said medium at a pressure not greaterthan 10-2 millimeters of mercury, means within said envelope forproducing a beam of particles having energies suicient to induce saidrotational transitions but insufficient to ionize said medium, meanslocated outside said envelope for applying a magnetic field to saidmicrowave resonant medium with the lines of said magnetic field parallelto the path of said beam to resolve the degeneracy of said rotationalstates, a cavity resonator within said envelope permeable to saidparticles and opaque to electromagnetic fields set up in said resonator,means for forming said particles into a beam of particles which entersaid resonator and collide with the microwave resonant medium containedtherein to induce said transitions with said resultant translation,means for coupling microwave input energy into said resonator at afrequency for which said medium is resonant, and means responsive tosaid translation for coupling microwave output energy from saidresonator at said resonant frequency.

13. Apparatus as claimed in claim 12, including means for controllingthe pressure of said microwave resonant medium.

14. Microwave apparatus comprising, an envelope containing a microwaveresonant medium at a pressure not greater than 10-2 millimeters ofmercury, a noble gas mixed with said microwave resonant medium, saidnoble gas being at a pressure at least two orders of magnitude greaterthan the pressure of said resonant medium, means within said envelopefor producing a vapor of particles, a cavity resonator within saidenvelope permeable to said particles and opaque to electromagneticfields set up in said resonator, means for forming said particles into abeam which enters said resonator and collides with the microwaveresonant medium contained therein, means located outside said envelopefor applying a magnetic field to said microwave resonant medium with thelines of said magnetic field parallel to the path of said beam, meansfor coupling microwave input energy into said resonator at a frequencyfor which said medium is resonant, and means for coupling microwaveoutput energy from said resonator at said resonant frequency.

15. A microwave amplifier comprising, a microwave resonant mediumcapable of exhibiting energy level transitions between quantum energyrotational states with resultant signal generation at at least oneresonant transitional frequency distinctively characteristic of saidmedium, means for producing a beam of particles having energiessufficient to induce said rotational transitions but insuicient toionize said medium, means for applying a magnetic field to said resonantmedium to resolve the degeneracy of said rotational states, means forcausing said beam particles to collide with said microwave resonantmedium to induce transitions with said resultant generation, means forapplying microwave input energy to said medium at a frequency at whichsaid medium is resonant and means for withdrawing microwave energy fromsaid medium at a rate not exceeding the sum of the rates at which energyis applied to and generated by said medium.

16. A microwave oscillation generator comprising, a microwave resonantmedium capable of exhibiting energy level transitions between quantumenergy rotational states with resultant signal generation at at leastone resonant transitional frequency distinctively characteristic of saidmedium, means for producing a beam of particles having energiessufiicient to induce said rotational transitions but insufficient toionize said medium, means for applying a magnetic field to said resonantmedium to resolve the degeneracy of said rotational states, means forcausing said beam particles to collide with said microwave resonantmedium to initiate the build-up of electromagnetic oscillations at afrequency at which said medium is resonant, and means for withdrawingelectromagnetic energy from said medium at said frequency at a ratewhich is slower than the rate of build-up of said electromagneticoscillations.

17. Apparatus as claimed in claim resonant medium comprising a fluid.

18. Apparatus as claimed in claim resonant medium comprising a gas.

19. Apparatus as claimed in claim resonant medium comprising a vapor.

20. Apparatus as claimed in claim 1, said microwave resonant mediumcomprising a solid at room temperature and pressure.

2l. Microwave apparatus comprising, a microwave resonant medium normallypresenting positive attenuation to electrical energy at frequencies forwhich said medium is resonant, means for introducing energetic particlesinto said microwave resonant medium having energies sufiicient to inducequantum energy rotational transitions in but insuicient to ionize saidmedium to cause said medium to present negative attenuation to microwaveenergy at a frequency for which said medium is resonant, and means forproviding electrical coupling to said medium.

22. Microwave apparatus comprising, a microwave resonant medium normallypresenting positive attenuation to electrical energy at frequencies forwhich said medium is resonant, means for introducing electrons into saidmicrowave resonant medium having energies suficient to induce quantumenergy rotational transitions in but insuicient to ionize said medium tocause said medium to present negative attenuation to microwave energy ata frequency for which said medium is resonant, means for providingelectrical coupling to said medium.

23. Microwave apparatus comprising, a microwave resonant medium normallyin a condition of thermal equilibrium, means for introducing energeticparticles into said microwave resonant medium having energies suficientto effect energy level transitions between quantum energy rotationalstates of, but insufcient to ionize, said medium thereby disturbing saidthermal equilibrium condition, and means for providing electricalcoupling to said medium.

24. Microwave apparatus comprising, a microwave resonant medium normallyin a condition of thermal equilibrium, means for introducing electronsinto said microwave resonant medium having energies sufficient to eectenergy level transitions between quantum er1- ergy rotational states of,but insufiicient to ionize, said medium thereby disturbing said thermalequilibrium con- 1, said microwave 1, said microwave 1, said microwavel@ dition, and' means for providing electrical coupling to said medium.

25. A microwave amplifier comprising, a microwave resonant mediumnormally presenting positive attenuation to electrical energy atfrequencies for which said medium is resonant, means for introducingcharge carriers into said microwave resonant medium having energiessuficient to induce quantum energy rotational transitions in butinsufficient to ionize said medium to cause said medium to presentnegative attenuation to microwave energy at a frequency for which saidmedium is resonant, and means for applying microwave input energy tosaid medium at a frequency for which said medium presents negativeattenuation and for deriving amplified microwave input energy from saidmedium.

26. A microwave amplifier comprising, a microwave resonant mediumnormally presenting positive attenuation to electrical energy atfrequencies for which said medium is resonant, means for introducingcharge carriers into said microwave resonant medium having energiessufiicient to induce quantum energy rotational transitions in butinsufficient to ionize said medium to cause said medium to presentnegative attenuation to microwave energy at a frequency for which saidmedium is resonant, means for applying microwave input energy to saidmedium at a frequency for which said medium presents negativeattenuation, and means for deriving amplified microwave input energyfrom said medium.

27. A microwave generator comprising, a microwave resonant mediumnormally presenting positive attenuation to electrical energy atfrequencies for which said medium is resonant, means for introducingenergetic particles into said microwave resonant medium having energiessufficient to induce quantum energy rotational transitions in butinsuficient to ionize said medium to cause said medium to presentnegative attenuation to microwave energy at a frequency for which Isaidmedium is resonant, and means for deriving microwave energy from saidmedium at a frequency for which said medium is resonant while saidmedium presents negative attennation.

28. A microwave generator comprising, a microwave resonant mediumnormally in a condition of thermal equilibrium, means for introducingenergetic particles into said microwave resonant medium having energiessucient to eiect energy level transitions between quantum energyrotational states of, but insufficient to ionize, said medium therebydisturbing said thermal equilibrium condition, and means for derivingmicrowave energy from said medium while said thermal equilibriumcondition is disturbed.

29. A microwave resonant medium atoms of which are in thermalequilibrium in magnetic substates of a ground state; and means forinducing energy level transitions between quantum energy rotationalstates in said medium for increasing the population of atoms in one ofsaid magnetic substates of the ground state at the expense of those inanother, said means including means for applying energetic particles tothe medium having energies sufficient to induce said rotationaltransitions but insufiicient to ionize said medium.

3G. A microwave resonant medium atoms of which are in thermalequilibrium in magnetic substates of a ground state; and means forinducing energy level transitions between quantum energy rotationalstates in said medium, said means including means for applying a beam ofenergetic particles to the medium having energies sufiicient to inducesaid rotational transitions but insucient to ionize said medium forincreasing the population of atoms in one of said magnetic substates ofthe ground state at the expense of those in another.

3l. Apparatus as claimed in claim 1 wherein said particles have energiessubstantially equal to the difference in energies between saidrotational states.

(References on following page) References Cited in the file of thispatent UNITED STATES PATENTS Pratt June 9, 1936 Pierce et al. Jan. 16,1951 Hershberger Apr. 24, 1956 Norton May 6, 1956 Dicke et al. June 5,1956 Dicke Sept. 11, 1956 12 2,837,693 Norton June 3, 1958 2,848,649Bryant Aug. 19, 1958 2,879,439 Townes Mar. 24, 1959 OTHER REFERENCESWeber: Reprint from I. R. E. Transactions, Professional Group onElectron Devices, published June 1953.

Gordon, Zeiger and Townes: Physical Review, v01. 95, page 282 (1954).

